The following references are considered by Applicants to be most pertinent to the present invention:
[A] Commonly assigned U.S. Pat. No. 5,159,513 discloses a magnetoresistive (MR) sensor which utilizes the spin valve effect. The sensor comprises a rectangular multilayered structure deposited on a glass or other suitable substrate. First and second ferromagnetic layers are separated by a thin copper spacer layer. The magnetization of the first layer is fixed in a direction across the width of the sensor by use of a hard magnetic material as the first layer or, if preferred, by use of an antiferromagnetic layer to pin the first layer by exchange coupling. The magnetization of the second layer is normally in a direction along the length of the sensor, but is free to rotate through an angle determined by the magnitude of the magnetic field being sensed. Hence, the sensor is an analog type device. PA1 [B] A paper entitled "Enhanced Magnetoresistance in Layered Magnetic Structures With Antiferromagnetic Interlayer Exchange," published in Physical Review B of the American Physical Society, Vol. 39, No. 7, p. 4828, (1989), notes that antiferromagnetic coupling between these layers results in a strong increase in magnetoresistance effect due to antiparallel alignment of the magnetizations in the ferromagnetic film layers, making this attractive for applications such as MR field sensors. PA1 [C] A paper entitled "Memory Implications of the Spin-Valve Effect in Soft Multilayers," published in J. Appl. Physics 69(8), Apr. 15, 1991, at p. 5760 et seq., describes a MR storage element that also has certain deficiencies hereinafter to be more fully described.
There is a need for a nonvolatile magnetic storage device which overcomes these deficiencies, in that it (1) achieves a direct current read without requiring state interrogation, (2) utilizes the full .delta.R/R of the magnetic layers to achieve high sensitivity, (3) improves read access time by obviating the need for lock-in detection, (4) enables reduction in the size of the spin valve cell primarily by reducing the width of the MR storage element and thus increases the signal level, and (5) enables high-speed write and erase operations.